With the growing interest in energy conservation, increasingly more machines, such as mobile industrial work machines or stationary power generation machines, are supplied with electric drive assemblies or systems for operating various tools or functions of the machine. Ongoing developments in electric drives have made it possible for electrically driven machines to effectively match or surpass the performance of mechanically driven machines while requiring significantly less fuel and overall energy. As electric drives become increasingly more commonplace with respect to such machines, the demand for more efficient generators and techniques for controlling same has also increased.
Among the various types of electrically driven machines available for use with such electric drives, switched reluctance (SR) machines have received great interest for being robust, cost-effective, and overall, more efficient. An SR machine is typically used to convert mechanical power received from a primary power source, such as a combustion engine, into electrical power for performing one or more operations of the machine. Additionally, an SR machine may be used to convert electrical power stored within a common bus or storage device into mechanical power. SR machines can similarly be used in conjunction with other generic power sources, such as batteries, fuel cells, and the like. Still further, SR machines can also be used with stationary machines having conventional power sources such as windmills, hydro-electric dams, or any other generic power source commonly used for stationary applications.
A typical SR machine essentially includes a multi-phase stator that is electrically coupled to an electric drive circuit, and a rotor that is rotatably positioned within the stator. In a generating mode of operation, the electric drive may be configured to receive any electrical power which may be induced by mechanical rotations of the rotor relative to the stator. Alternatively, in a motoring mode of operation, the electric drive may be configured to selectively source current through the phases of the stator so as to cause electromagnetic interactions between the stator and rotor poles and rotate the rotor relative to the stator at a desired torque and/or speed. More specifically, the current through each phase of the stator is typically pulsed or chopped by gates or switches of the electric drive at a predefined rate or switching frequency.
Using conventional techniques, however, often results in inconsistent levels of current or uneven current ripple in the pulsed phase current. As current ripple in the phase current directly affects the electromagnetic interactions between the stator and rotor poles, current ripple also leads to undesirable machine behavior and performance. In some modifications, the switching frequency was permanently increased to help mitigate current ripple. However, the amount of increase in the switching frequency needed to sufficiently overcome the adverse effects of current ripple has been found to be too large and burdensome to the SR machine. Specifically, such increases in switching frequency have been found to exhibit undesirable increases in power loss of the electric drive.
Accordingly, there is a need for improved SR machine controls which overcome the deficiencies identified above and reduce power losses in the electric drive. Specifically, there is a need to reduce current ripple in the chopped current supplied to each phase of SR machines without significantly increasing the switching frequency to offset the current ripple.